Ion exchange softeners and deionizers of the fixed bed type have been used for many years in industrial and commercial applications. The development of "moving bed" or continuous ion exchange systems has resulted in significant efficiencies in industry. In this type of a system, the major functions, loading on the one hand and regenerating, backwashing and rinsing on the other, were separated into different vessels and the resin moved from one vessel to the other and back. However, these systems require high-pulse pressure in order to move the resin beds and, as a consequence, high pressure vessels which meet ASME codes had to be used with the system, thus, may limit their application. Typical moving bed systems of this type are shown in the Higgins U.S. Pat. No. 3,677,937 issued July 18, 1972; U.S. Pat. No. 3,580,842 issued May 25, 1971; U.S. Pat. No. 3,984,313 issued Oct. 5, 1976; U.S. Pat. No. 3,579,322 issued May 18, 1971; U.S. Pat. No. 3,492,092 issued Jan. 27, 1970; and U.S. Pat. No. 2,815,322 issued Dec. 3, 1957. The major drawback of the Higgins loop was the requirement to move the entire resin inventory, often tons of resin, in a single pulse movement even though only about one-sixth of the resin volume was transferred from one vessel to the other. The high pressure required to move the resin also caused turbulence, particularly in the larger apparatus, in the resin bed, thereby upsetting the laminar resin/liquid flow which should flow in a laminar relation for maximum efficiency. Four large butterfly valves had to be used between the vessels in order to allow for the movement of these massive amounts of resin.
Some of these problems were overcome by the earlier U.S. Pat. Nos. 4,228,001 to Carlson and 4,208,904 wherein a low pressure system was disclosed in which the resin inventory is moved in several phases. This was accomplished at low pressure by moving a small amount of resin from the loading vessel to one of the support vessels and then moving a corresponding amount of clean resin into the bottom of the loading vessel from another support vessel. This eliminated the requirement for high pressure vessels, thus, reducing costs considerably. The large butterfly valves also were eliminated since only a small volume of resin was moved at one time and the interfaces in the regeneration column were not disturbed by the gentle movements of the resin. The system could be shut down for long periods of time, since the only resin flow required was at the top of the loading vessel and at the bottom of the regeneration column, either as a single movement, or as two separate movements. However, this apparatus required three support vessels to make up the regeneration section which are costly to make and had to be linked together to allow for resin transfer.